The present invention relates to a liquid-crystal display device of active matrix driving type utilizing thin film transistors (hereinafter referred to as “TFT”) and a manufacturing method therefor.
The liquid crystal display device of active matrix type is a display device in which a switching element is provided in each of plural pixel electrodes arrayed on a display panel in matrix-shape. The liquid crystal display device of active matrix type is capable of displaying images with better contrast than a liquid-crystal display device of simple matrix type employing a time sharing driving method, and the active matrix driving method is an indispensable technique for color display.
A conventional liquid crystal display device of active matrix type comprises a first insulator substrate (TFT substrate) having on one side of the surfaces a plurality of scanning signal bus-lines spaced at a given distance extending in a first direction, a plurality of image signal bus-lines spaced at a given distance extending in a second direction intersecting with the scanning signal bus-lines, TFT's placed near the intersecting points of the scanning signal bus-lines and the image signal bus-lines, their gate electrodes being connected to the scanning bus-lines, their drain (or source) electrodes being connected to the image signal bus-lines, pixel electrodes each connected to the source (or drain) electrode of a TFT placed in each of the areas determined by the adjacent two scanning signal bus-lines and the adjacent two image signal bus-lines, a second insulator substrate (counter substrate) having counter electrodes formed on one side of the surface placed so as to face the counter electrodes to the pixel electrodes, and a liquid-crystal layer interposed between the first insulator substrate and the second insulator substrate.
A large problem to be solved in order to realize a liquid-crystal display device of active matrix type is to decrease its manufacturing cost, and more particularly to decrease the manufacturing cost of the TFT substrate. A method concerning this problem is described in Japanese Patent Application Laid-Open No. 62-32651 (1987) where the number of photo-lithography processes is decreased by forming the semiconductor layer and the gate insulating film using the same pattern at a time
However, in the method described in Japanese Patent Application Laid-Open No. 62-32651 (1987), the semiconductor layer and the gate insulating film are patterned in island-shape in the same shape on the portion of the thin film transistor. Therefore, when the drain electrode or the source electrode is not formed in a normal pattern, it increases the probability of the occurrence of wiring faults where both electrodes and the gate electrode short-circuit along the side surface of island-shaped pattern.
Further, since in the intersecting point portion of the scanning signal bus-line and the image signal bus-line both signal bus-lines are separated by an island-shaped pattern of the semiconductor layer and the gate insulating film formed in the same shape, the image signal bus-line (drain bus-line) needs to be wired in a manner such that the wire climbs over a large step at the portion of the island-shaped pattern. This leads to a disadvantage in that a break in the wire is apt to occur at the step near the pattern.
On the other hand, in a liquid-crystal display device of active matrix type, in order to prevent degradation in image quality due to a parasitic capacitor between the gate and the source in a TFT, it is commonly performed to provide capacitor elements (storage capacitor Cstg) between the pixel electrode and the counter electrode, and an electric potential line other than the scanning signal bus-line, or to provide capacitor elements (storage capacitor Cadd) between the pixel electrode and the scanning signal bus-line in the precedent stage. The liquid-crystal display device of active matrix type having the storage capacitor Cstg is described in, for example, Japanese Patent Application Laid-Open No. 3-149520 (1991), and the liquid-crystal display device of active matrix type having the storage capacitor Cadd is described in, for example, Japanese Patent Application Laid-Open No. 5-204337 (1993).
The liquid-crystal display device of active matrix type having the storage capacitor Cstg is capable of high speed scanning at driving the scanning signal bus-lines and is suitable for large screen size and high resolution, since the storage capacitor Cstg is not connected to the scanning signal bus-line. However, the storage capacitor Cstg requires a counter electrode and a bus-line for applying electric potential other than the scanning signal bus-line. This decreases the production yield which depends on the aperture ratio of the pixel and the cross over area of the bus-lines.
On the other hand, the liquid-crystal display device of active matrix type having the storage capacitor Cadd has an advantage in that no newly added bus-line for applying electric potential to the storage capacitor Cadd is necessary, and, therefore, the device has merit in the aperture and in the production yield.
However, since the storage capacitor Cadd is connected to the scanning signal bus-line, it is difficult to perform high speed scanning. Therefore, there is a disadvantage in that it is difficult to realize a high resolution and large screen display.
In order to decrease the burden on the TFTs or to decrease cost for forming the driving circuit of image signal bus-lines, alternating driving of the counter electrode is generally employed to lower the image signal voltage. In this driving method, it is necessary to lower the resistivity of the scanning signal bus-line and the counter electrode in a case of employing the storage capacitor Cadd since the image quality is affected by the product of the capacitance and the electrode resistivity connected to the scanning signal bus-line, the resistivity and the capacitance of the counter electrode and so on.